Dirty diesel oil is
a real bummer. It can clog your fuel filters, damage your injectors, and
in serious cases render your engine as dead as a doornail. But while most
folks rely on their filters and fuel/water separators to sieve the junk
out, the best way is to clean the fuel while it’s still in the tanks.
Engine guys call this "polishing," and thanks to new technology
on the market, it’s a procedure now available even to owners of smaller
yachts. Once you have a polisher installed on your boat, keeping your
fuel clean is easier than keeping your bowrails shiny.

Keeping your fuel well-shined
will prevent sludge from building up in your tanks and prevent algae from
thriving by getting rid of stray water; algae (actually hydrocarbon-eating
microbes) need both fuel and water to survive. During its voyage from
the refinery to the fuel-dock nozzle, diesel fuel picks up water and dirt
from every tank and pipeline it passes through. (Condensation forming
in half-empty storage tanks adds more water all the time.) Once onboard,
most of the junk settles to the bottom of your tanks, where the microbes
multiply in the layer of water under the fuel. When the sludge level rises
high enough, the gunk can find its way into the fuel pickup. That’s
why good filters make good shipmates.

While in-line fuel filters
and water separators are fine as a last defense for keeping sludge out
of the engines, they have three drawbacks: First, they need attention
and maintenance–draining the sediment bowls, changing the filter
elements, and so forth. Otherwise, they can clog and stop the fuel flow.
This ain’t rocket science, but it’s a chore a lot of skippers
tend to overlook.

Second, in-line filters
can’t remove the tiniest bits of junk and microparticles of water.
Since they’re mounted in the fuel lines, filters must be able to
strain the diesel while allowing enough flow to keep the engines running.
With twin-engine setups of even modest horsepower, this can mean a total
flow rate of 75 gph or more. Most filter elements are rated between 10
and 30 microns. (A micron is one-millionth of a meter.) Particles smaller
than that can pass through. Very fine filter elements generally go hand-in-hand
with restricted flow rates. Fuel polishers, on the other hand, operate
at lower gph rates and with much finer filtration, so they won’t
starve engines of fuel.